What Causes Swelling Of The Pectoral Muscle?

My boyfriend's left pectoral muscle is entirely swollen. He has been on antiobiotics, but it has not helped at all. The doctor (urgent care) at first said he thought it was a cellulitis; however, when given the first meds nothing changed. He went again only to be put on 2 different medications and after three days still no change. He goes in tomorrow for an ultrasound. What could be the problem?